The 37-year-old former England captain, who has joined the wealthy capital club on a five-month deal, replaced Javier Pastore in the 75th minute to the delight of a roaring home crowd.
Beckham, who sat on the bench after the packed stadium welcomed him as the sound system played The Beatles song "Hey Jude", immediately produced a trademark long pass to Ezequiel Lavezzi before joining in with a sterling defensive performance.
"He has brought his passing skills, his experience and tactical solutions," PSG coach Carlo Ancelotti told reporters.
"He only played 20 minutes tonight but he showed some good things."
The visiting team dominated possession and had the best chances throughout but Italy reserve keeper Sirigu made a string of superb saves to preserve the leaders' advantage until Zlatan Ibrahimovic sealed victory with the second goal from close range in stoppage time.
PSG have 54 points from 26 games, three ahead of Olympique Lyon, who beat Lorient 3-1 at home thanks to Lisandro Lopez, Rachid Ghezzal and Arnold Mvuemba's goals. Marseille stayed third on 46.
The hosts seemed started well and after two minutes Lucas hit the post following Ibrahimovic's back-heeled volleyed pass.
They were quickly rewarded with Nkoulou's own-goal but then let the visitors dominate possession, banking on in-form Ezequiel Lavezzi's pace to create chances.
Marseille struggled to unsettle their hosts' tough defence for half an hour, until Foued Kadir produced a fine solo effort down the right, only to see his powerful shoot parried away by Sirigu.
Marseille continued to push hard early in the second half but could not beat the inspired Sirigu who stopped Gignac's 61st-minute overhead kick and Andre Ayew's neat header.
Ghezzal tapped the ball in five minutes into the second half and Lyon wrapped up the win thanks to Arnold Mvuemba's last-gasp goal.
"I've waited for this goal for a long time now," winger Ghezzal told French channel beIN Sport
